When England adopted the Gregorian calendar in 1752, some 170 years after it was introduced by Pope Gregory XIII, Benjamin Franklin wrote, "It is pleasant for an old man to be able to go to bed on Sept. 2, and not have to get up until Sept. 14." Indeed, nearly two weeks evaporated into thin air in England when it transitioned from the Julian calendar, which had left the country 11 days behind much of Europe. Such calendrical acrobatics are not unusual. The year 46 B.C., a year before Julius Caesar implemented his namesake system, lasted 445 days and later became known as the "final year of confusion."...